1. Field of the Invention
The present invention relates to an ultrasonic motor using vibrations of a vibrator such as a piezoelectric element.
2. Description of the Related Art
Recently, an ultrasonic motor using vibrations of a vibrator such as a piezoelectric element is receiving attention as a novel motor which replaces an electromagnetic motor. The ultrasonic motor is superior to the conventional electromagnetic motor because it can obtain high thrust at low speed without any gear, have high retention at power off, long stroke, and high resolving power, is very quiet, does not generate magnetic noise, and is free from the influence of magnetic noise.
In the ultrasonic motor, an ultrasonic vibrator is pressed against a driven member serving as a relative motion member via a driving member serving as a frictional member. When the ultrasonic vibrator vibrates, a frictional force is generated between the driving member and the driven member to drive the driven member.
For example, the following ultrasonic motor is known. In this ultrasonic motor, alternating signals of two phases are applied to the ultrasonic vibrator to simultaneously excite longitudinal and flexural vibrations in the ultrasonic vibrator, thereby generating elliptic vibrations in the driving member. A driving force is obtained from the elliptic vibrations to relatively move the driven member.
As a technique concerning the ultrasonic motor, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-304425 discloses the following technique. In an ultrasonic motor operating method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-304425, alternating voltages of two phases having a predetermined phase difference and predetermined driving frequency are applied to an electromechanical transducer for driving. As a result, two different vibration modes are simultaneously generated to cause almost elliptic vibrations at the output terminal. According to this operating method, a press force for pressing the output terminal of the ultrasonic vibrator against the driven member is set based on a signal output from an electromechanical transducer for vibration detection so that mechanical resonance frequencies in the two different vibration modes coincide with each other.
The technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-304425 provides an ultrasonic motor which simultaneously generates a plurality of vibration modes and can efficiently generate each vibration mode to stably obtain a high motor output.
As a driving characteristic of the ultrasonic motor, driving becomes unstable in a low-speed region (driving phase difference is almost 180°). Under the circumstances, burst driving (intermittent driving) is known as a technique of stably driving an ultrasonic motor at low speed. In the burst driving, a driving signal applied to an ultrasonic vibrator is cyclically enabled and disabled. However, the burst driving generates abnormal noise upon driving.
The following driving method is proposed as a technique for reducing generation of abnormal noise. More specifically, a plurality of driving phase differences are set, and one of them is set to have a driving speed of 0. These driving phase differences are cyclically switched to drive an ultrasonic motor. Also in this case, the ultrasonic motor can be stably driven at low speed, similar to the foregoing burst driving. This driving method can reduce generation of abnormal noise, compared to the burst driving. However, further reduction of abnormal noise is required.